Have you ever wondered of an easier way of working out the reverse subnet mask for your ACL’s? Instead of just opening up your subnet calculator you can do it all in your head just like this.
For example a 27 prefix translates to 27 bits so with a reverse mask of

A full 8 bit byte is 255 minus your 224 equals 31 so there you have it your reverse mask of 224 is 31

Let’s look at one more so take 26 prefix is with a reverse mask of

Again take 255 minus your 192 equals 63.